EP0634759A2 - Semi-hard and deformable iron based permanent magnet alloy - Google Patents

Abstract

To produce a semihard (half-hard, medium-hard), deformable iron-based permanent magnet alloy, an austenitic alloy is used, which is metastable owing to the addition of more than 5% of nickel and/or manganese, and which, in order to set a coercive field strength from over 40 up to 100 A/cm and a remanent induction (remanent flux density) of more than 0.8 T, contains a proportion of from 4.5 to less than 25% of cobalt.

Description

The invention relates to a semi-hard, deformable permanent magnet alloy, which essentially contains iron, additions of over 5% nickel and / or manganese together and at least one additional element for increasing the transformation temperature into austenite (As) in such amounts that a metastable austenitic alloy results, which contains a mixed structure of an alpha phase (martensite) and a gamma phase (austenite) by cold working and subsequent heat treatment.

Alloys with 68 to 77 wt .-% iron, 9 to 20 wt .-% chromium and 13 to 23 wt .-% nickel are from G. Rassmann and O. Henkel (NACHRICHTENTECHNIK, 11 (1961), pp. 307-313 ) have been examined. After homogenization, these alloys are austenitic and unstable to deformation, i. H. Cold forming can convert measurable amounts of austenite into ferrite. The alpha phase (ferrite, martensite) generated by cold working can then be converted back to austenite by heat treatment. This thermomechanical treatment enables a coercive field strength in the range from 50 to 900 A / cm to be set with a remanent induction Br between 0.06 and 0.3 T.

The object of the present invention is to create a semi-hard deformable permanent magnet alloy which has a coercive field strength Hc in the range from more than 40 to 100 A / cm and a remanent induction Br above 0.8 T. An alloy with such magnetic properties is particularly useful when used as an anti-theft strip needed. Here, together with the actual soft magnetic security strip, it serves to validate it at the cash register by magnetization, so that it can then no longer trigger an alarm. However, the alloy is also suitable for other applications in which the requirements for coercive field strength and a minimum residual induction of over 0.8 T have to be met.

In addition to the known alloys mentioned above, with which at most about 0.3 T of residual induction can be achieved, other semi-hard deformable permanent magnet alloys are known. For example, from CONCISE ENCYCLOPEDIA OF MAGNETIC & SUPERCONDUCTING MATERIALS by J. Evetts (1992), pages 197 - 200, an alloy with 40 to 78% Fe, 2 to 25% Co and 20 to 35% Cr can be found for adjustment the magnetic properties by quenching from a high temperature of over 1200 ° C is present as ferrite, which then undergoes spinodal segregation into an iron-rich and a chromium-rich phase at slow cooling rates of about 0.1 ° C / h. The magnetic properties can be adjusted by this spinodal separation. A disadvantage of the properties of an alloy intended here is the heat treatment, which is difficult to adjust the coercive field strength, in particular in the case of large batches, it not always being possible to ensure that the same temperature actually exists in every part of a furnace filling.

Furthermore, it is known from CONCISE ENCYCLOPEDIA OF MAGNETIC & SUPERCONDUCTING MATERIALS by J. Evetts (1992), pages 211 to 213 to provide an Fe-Co-V alloy with approximately 50% cobalt and 6 to 16% vanadium. This alloy is also austenitic metastable in certain compositions, such as the known one mentioned above Fe-Cr-Ni alloy and can be magnetically hardened by cold working with subsequent heat treatment. The disadvantage of these alloys, however, is the high cobalt content, which is a prerequisite for magnetic hardenability and, because of the high price for cobalt, leads to a relatively expensive alloy that is less economical than the alloy claimed here, for example for the purpose of the anti-theft devices.

Alloys are known from CONCISE ENCYCLOPEDIA OF MAGNETIC & SUPERCONDUCTING MATERIALS by J. Evetts, (1992) pages 475 - 478, as Table 1 shows on page 477, which contain nickel and / or manganese with other additives to increase the transformation temperature into austenite contain. However, these alloys are also uneconomical because of the high cobalt content and also do not meet the requirement of a coercive field strength in the range from 50 to 100 A / cm, as is required for the intended purposes here.

The effects of the additional elements for increasing the transformation temperature into austenite are known from TRANSACTIONS OF THE METALLURGICAL SOCIETY OF AIME, Vol. 227 (1963), pages 884-890, so that the person skilled in the art can choose the amount and the composition of these additives, to obtain a metastable austenitic phase in which the transformation temperature from austenite to martensite (Ms) is below room temperature and at the same time a sufficiently high temperature can be set for the transformation of martensite (alpha phase) to austenite (gamma phase) (As) .

As an exemplary embodiment, an alloy with 67Fe-14Cr-7Ni-5Mo-10Co was hot-rolled at 1100 ° C. to 5 mm, then annealed at 1100 ° C for 1 h and quenched in water. In this state, the alloy is paramagnetic and austenitic. This alloy was then rolled to 0.5 mm, corresponding to 90% cold working. In this state, the alloy is ferromagnetic by converting the gamma to the alpha phase. The magnetic values are Br = 0.4 T for the remanent induction and Hc = 25 A / cm for the coercive field strength. By tempering in the temperature range of 400 to 600 ° C for 1 min to 24 h, Br and Hc can be increased considerably. After a heat treatment of 3 h at 500 ° C there was a coercive field strength Hc = 70 A / cm and a remanent induction Br = 1.1 T. Further compositions with a cold deformation of 86% or 90% and an annealing at different temperatures are shown in tables 1, 2 and 3.

Table 1 shows alloys containing iron, Ni, Cr, Mo, Mn and partly Co and Ti. It can be seen that the magnetic values Br for the remanence induction and Hc for the coercive force are in the required range, provided the Co content is above 4.5% and the other additives are selected so that a metastable austenitic alloy results after the homogenization annealing . It can also be seen that particularly advantageous magnet values result if the degree of cold deformation is at least 90%.

The same is shown in Table 2, in which the temperature of the final annealing was increased to 520 ° C., while Table 3 has the same alloys as in Table 2 for the test object and shows the influence of the temperature of the heat treatment.

The examples further show that a particularly advantageous composition is obtained if the cobalt content is greater than 4.5 and at most 12% and if a cold working of over 80% is carried out.

Since metastable austenitic alloys are characterized by the fact that the austenite can be converted to martensite either by cold working or by cooling below the transformation temperature of the austenite to martensite with the alpha phase, the conversion to martensite cannot be achieved by cold working or by cooling , if the proportions of Ni and Mn and the proportions of the other additives become too large. You then have to deal with stable austenitic alloys. TABLE 1 Annealing at 520 ° C, 3 h with different cold working Alloy composition Magnetic values KV 86% KV 90% Co Ni Cr Mon Mn Ti Br Hc Br Hc % % % % % % T A / cm T A / cm 9298 11.5 6.1 14.0 4.5 0.7 1.15 66 1.22 64 9353 9.8 7.0 12.8 4.8 0.6 0.3 0.99 56 1.22 75 9270 9.7 7.0 12.9 5.0 0.6 1.13 62 1.23 67 9349 9.1 7.0 12.8 4.5 0.5 1.05 46 1.24 60 9303 9.0 7.0 13.5 4.5 0.7 0.98 64 1.10 57 9302 8.0 8.0 11.5 3.8 0.6 0.99 40 1.14 37 9299 8.0 8.0 13.5 4.5 0.7 0.86 74 1.10 64 9350 4.5 6.9 13.1 4.5 0.5 0.90 33 1.12 38 9351 4.5 7.9 12.1 4.5 0.5 0.94 36 1.16 39 9300 4.5 10.0 13.0 4.5 0.7 0.38 95 0.69 88 9301 11.8 12.5 4.5 0.6 0.14 153 0.28 139 Final annealing at 520 ° C, 3 h, cold deformation approx. 90% Alloy composition Magnetic values Co Ni Cr Mon Mn Ti Br Hc HV % % % % % % T A / cm 0.5 9353 9.8 7.0 12.8 4.8 0.6 0.3 1.22 75 736 9349 9.1 7.0 12.8 4.5 0.5 1.24 60 613 9350 4.5 6.9 13.1 4.5 0.5 1.12 38 552 9351 4.5 7.9 12.1 4.5 0.5 1.16 39 560 Final annealing 3 h at different temperatures, cold deformation 90% Br (T) Hc (A / cm) 480 ° C 500 ° C 520 ° C 540 ° C 480 ° C 500 ° C 520 ° C 540 ° C 9349 1.21 1.23 1.24 1.07 44 51 60 72 9350 1.03 1.05 1.12 1.09 35 37 38 40 9351 1.10 1.12 1.16 1.14 34 38 39 44 9353 1.13 1.17 1.22 1.14 52 62 75 81

Claims (4)

Semi-hard, deformable permanent magnet alloy, which essentially contains iron, additions of more than 5% nickel and / or manganese and at least one additional element to increase the transition temperature to austenite (As) in such quantities that a metastable austenitic alloy results Cold forming and subsequent heat treatment contains a mixed structure of an alpha phase (austenite) and a gamma phase (martensite), characterized in that they are used to set a coercive field strength (Hc) in the range from more than 40 to 100 A / cm and a remanence induction (Br) over 0.8 T additionally over 4.5 to less than 25% by weight cobalt and at least one of the additional elements Cr, Cu, Mo, W, Si, V, Nb, Al, Ti, Ta, Zr, ( C + N) contains. Semi-hard deformable permanent magnet alloy according to claim 1, characterized in that the cobalt content is over 4.5 to a maximum of 12% by weight. Semi-hard deformable permanent magnet alloy according to claim 1, characterized in that the metastable austenitic alloy cooled from the austenitic region (gamma phase) is subjected to a cold deformation of over 80% before the subsequent heat treatment for magnetic hardening. Semi-hard deformable permanent magnet alloy according to claim 3, characterized in that the alloy is subjected to a cold deformation of at least 90%.